Refugee camps are prone for easy spread of infections of various kinds and tuberculosis (TB) is no exception. Refugees with renal failure are often a vulnerable group because they are immunocompromised due to reasons such as poor nutrition, overcrowding and immune suppression due to renal failure. Latent pulmonary TB is a particular problem in this patient population as it is not easily diagnosed and has immense potential for spread. Tuberculin Skin Test (TST), although easy to perform and is cost-effective, suffers from the limitations of giving false positive results due to cross-reaction with the vaccination. Chest radiography though cheap, has not yet been validated in refugee populations for this purpose. Sputum analysis shows promise due to ease of performing but again has not been validated in refugees. Newer assays such as IF-γ show great promise but needs large scale studies for validation and cheaper assays need to be developed for use in resource poor refugee setting. In short, an ideal tool for effective screening of latent TB in refugees with renal failure is lacking. Future studies are required to identify this ideal tool.

Infections of various kinds are very common in refugee camps. Diarrheal diseases, malaria, tuberculosis (TB), influenza, Lyme's disease, Brucellosis More Details, leptospirosis, etc., are common infections that affect refugee populations. This is especially true among refugees with renal failure. The reasons that predispose refugees to infections are overcrowding, poor sanitation and waste disposal, unprotected food and water, and unhygienic living conditions which are inevitable features of refugee camps. Also, renal failure causes immunosuppression in these patients. Interestingly, many of these infections are hidden or latent and are not clinically apparent. Such latent infections not only cause morbidity to the affected individuals but also to the people who live near them.

TB is a common infection that affects refugees. Owing to overcrowding in refugee camps, TB spreads easily via droplet infections. It is interesting to note that TB is latent in many of the refugees. The World Health Organization has estimated the global prevalence of latent TB infection in 1997 to be 35% for Africa, 44% for Southeast Asia and 15% for Europe. [1] In countries like the United States (US), Norway, and United Kingdom where a sizeable refugee immigration happens, the rates of TB in the native population is usually low but the immigrant population has a high prevalence, which poses a significant public health problem of potential spread in the native population. It is reported that in the US, the rate of active tubercular infection is nearly 9.7 times higher in foreign-born persons than among US-born persons. [2] The incidence of TB in Norway is generally low (6.3/100,000 population in 2006), but high among immigrants from countries where TB is endemic. [3] The enormous pool of persons with latent TB challenges the control of this disease in low endemic countries. National guidelines for prevention and control of TB in most low endemic countries therefore, recommend targeted tuberculin testing and treatment of latent infection. [4] Interestingly, majority of these infections are believed to be reactivations of tubercular infection that were acquired before immigration which highlights the inadequacy in the current preventive strategies. [2] There are continuing efforts to devise effective screening tools to diagnose latent TB among refugees as this can be a significant public health problem. Traditionally, Tuberculin Skin Test (TST) and chest radiography have been used to screen refugees. TST is slowly being replaced by newer interferon-gamma (IFN-γ) release assays (IGRAs) which have the potential to address many of the shortcomings of TST in refugee populations such as eliminating false positive results caused by Bacillus Calmette-Guerin (BCG) vaccination, distinguishing between tuberculous and non-tuberculous mycobacterial infection (NTB), and providing test results after a single patient visit. [5],[6],[7] The objective of this review is to enumerate the various techniques that are currently available to screen for latent TB in refugees, compare and contrast each of these techniques with the other in terms of cost, validity, and reliability and finally reach an optimal conclusion as to what will be the best among the available techniques.

TST: Advantages and Disadvantages

Traditionally, TST has been used as a screening tool for latent TB. The advantages of this test are that it is cheap, easy to perform, safe, causes minimal discomfort to the patient and is easily available. These characteristics make it the first choice in a setting where there are hundreds of refugees and many have to be screened. In many countries, the national guidelines recommend targeted TST in refugees and to treat those who have a positive TST test. [4] However, TST has significant limitations as has been already enumerated. False positive results due to prior BCG vaccination, other tubercular infections and false negativity in immunocompromised people and those with miliary TB are the common short comings of TST.

Chest Radiography in Screening

An interesting study was conducted where health-care workers who had a positive TST as part of their pre-employment evaluation were retested with chest radiography to look for the agreement between the two tests. A total of 2586 asymptomatic individuals with positive TST results were evaluated to determine the frequency of detection of evidence of active TB or latent TB infection (LTBI) and the spectrum of imaging findings. The study concluded that universal chest radiography in a large pre-employment TB screening program was of low yield in the detection of active TB or increased LTBI reactivation risk, and it provided no assistance in deciding which individuals to prioritize for LTBI treatment. [8] However, a similar study that has evaluated the validity of chest radiography as a screening tool in refugee populations is not available. The results may or may not be comparable with the study that has been described above for reasons that refugees may not be as healthy as health-care workers due to what is commonly called as "healthy worker effect". So, the effectiveness of chest radiography as an effective screening tool in refugee population is still not clear.

Sputum Analysis as a Screening Tool for Latent TB

A prospective multicenter cohort study comprising 1,171 individuals who were seropositive for human immunodeficiency virus (HIV) but did not have AIDS at the time of enrollment and 182 HIV-seronegative controls, were studied by means of routine induced-sputum analysis in an attempt to detect occult TB. [9] The study concluded that routine induced-sputum analysis is not an effective strategy for screening HIV-infected asymptomatic subjects for TB before the onset of cli­nically recognizable disease activity. A similar study assessing the validity of sputum analysis as a screening tool for latent TB in refugees is not available. Also, it is not possible to extrapolate the results we got from the HIV infected patients to healthy seronegative refugee populations. Thus, sputum analysis also lacks supporting data to validate its usefulness as a screening tool for latent TB in refugees.

Newer Assays

In vitro assays based on cellular production of IFN-γ in response to the Mycobacterium tuberculosis- specific antigens ESAT-6 and CFP10 have recently been developed. These protein antigens are present in all species of the M. tuberculosis-complex (including Mycobacterium bovis), but absent in all vaccine strains of M. bovis-BCG and most non-tuberculosis mycobacteria, except Mycobacterium marinum, Mycobacterium zulgai, and Mycobacterium kansasii. These tests can therefore diagnose infection with M. tuberculosis with a higher specificity. [7],[10] One of the IFN-γ release assays, QuantiFERON®TB Gold (QFT), offers logistic advantages and may be suitable for routine screening. The assay has been tested in numerous contact investigations, among patients with TB disease and exposed health-care workers, but data are limited for immigrant screening. [11],[12] Among 100 immigrants from high prevalence countries attending an outpatient clinic in Italy, 44% and 15% were positive with TST (≥10 mm) and QFT, respectively. [13]

How is QFT done?

For the QuantiFERON®TB Gold in-tube-test, 1 mL of venous blood is drawn into one tube pre-coated with synthetic peptide antigens and one tube without antigens (negative control sample) and transported the same day to the analysis center. Samples were incubated, processed and stored in accordance with the manufacturer's instructions following which the harvested plasma is subjected to Enzyme-Linked Immunosorbent Assay (ELISA) analysis, including IFN-γ standard for quantification. The quality of all laboratory analyses and calculation of the results will have to be controlled by using QFT analysis software. A sample is considered positive if it exceeds the standard cut-off value at 0.35 IU IFN-γ/ mL. All positive results are then confirmed by re-analysis of the same plasma sample before reported as positive. If it is not possible to reproduce a positive result, the QFT result will be reported as non-conclusive. [14]

Comparison between TST and QFT

A study from Norway [14] where 1000 asylum seekers who were more than 18 years of age were tested with both TST and QFT. IFN-γ cut-off value was 0.35 U/mL and a skin in duration of > 6 mm was considered a positive TST. Fifty percent tested positive on TST and 29% tested positive with QFT. Among the TST-positive participants, 50% were QFT negative, whereas 7% of the TST-negative participants were QFT positive. There was a significant association between increase in size of TST result and the likelihood of being QFT positive. Agreement between the tests was 71- 79% depending on the chosen TST cut-off and it was higher for non-vaccinated individuals. The agreement as assessed by Kappa statistics was also similar between QFT and chest X-ray. The study finally concluded that by using QFT instead of TST, further follow-up could be avoided in nearly 43% of asylum seekers. Further, QFT alone will be a good screening tool or it can be used as a tool to retest those individuals who were positive with TST.

QFT in Special Subcategories of Patients

Drug users

In an interesting study, TST was tested against QFT in 48 HIV-seronegative former drug users. The agreement between TST and the QFT assay for latent TB was 73% (κ = 0.45). The study concluded that QFT was a valid screening tool and can be used in lieu of TST in this subgroup. [6]

High HIV prevalent setting

In a study from Trinidad, investigators compared the QFT assay and TST in screening/diagnosis of latent TB infection (LTBI) among individuals in Trinidad and Tobago at high-risk for TB. [15] A total of 560 individuals [TB patient contacts, HIV patients, health-care workers, prison inmates, and TB patients (controls)] were recruited for the study. The QFT detected LTBI in 51% of the subjects (with most positive results occurring among the control group) whereas the TST detected it in 39.4% (P = 0.001). Overall, the QFT assay detected LTBI more frequently than the TST among all subjects except the control group, where detection favored the TST. The QFT assay produced indeterminate and non-reactive results in some HIV patients but required less turn around time than the TST (23.3 h versus 70.2 h; P < 0.0001). The cost of TST is lesser per subject than the QFT assay (US $3.70 versus US $18.60; P = 0.0008). The investigators concluded that QFT assay costs more but had a higher detection rate among most target groups and required less turn around time than the TST. However, its sensitivity was lower among immunocompromised subjects. Therefore, the QFT assay should be used with caution for LBTI screening/diagnosis in resource-poor, high-HIV prevalence settings such as Trinidad and Tobago.

Children

QFT was compared with TST in diagnosing TB infection in children, and to analyze discordant results. This prospective study included 98 children from contact-tracing studies and 68 children with TST >/= 5 mm recruited during public health screenings. [16] The study concluded that the use of QFT was helpful for the diagnosis of TB infection, avoiding cross-reactions with BCG immunization. Again, this study involved healthy children. Extrapolation of the findings of this study to refugee children needs validation in future studies.

ELISPOT: Another IFN- γ Assay

In addition to QFT, ELISPOT is the other commercially available IFN-γ assay that has been well studied. Like QFT it involves a similar type of blood test and an assay for IFN-γ and newly identified tubercular proteins like Rv1985c.[17] Like QFT it is highly specific for active infection and usually tests negative in BCG-vaccinated people and in the presence of other mycobacterial infections. [17]

Comparison between ELISPOT and TST

In a study conducted on 69 rheumatic patients where TST and ELISPOT (Enzyme Linked Immuno spot assay) was applied to all the participants in the study, 17 (25%) had a positive TST response and 15 (22%) had a positive ELISPOT response. Among the patients with a positive TST result, eight had a positive and nine a negative ELISPOT response, whereas among the 49 patients with a negative TST result, 42 were ELISPOT negative, but seven (14%) were ELISPOT positive, with three indeterminate results. The agreement between the two tests was poor. Thus, the ELISPOT-IFN- γ assay performed better than the TST in recognizing patients with latent TB, thus reducing the number of patients submitted to isoniazid prophylaxis on one hand, and on the other hand, since the assay is less biased by immunosuppressive regimens than TST, recognizing LTBI patients among those with a negative TST response. [18] Although this study shows promise that ELISPOT is indeed a better assay than TST, it is limited by the fact that this was conducted on rheumatic patients. Thus, extrapolation of these findings to a refugee population will be difficult. A similar data from refugee population is lacking and it is for future studies to analyze this possibility.

Comparison between ELISPOT, QFT and TST

A prospective study was conducted involving 393 consecutively enrolled patients who were tested simultaneously with ELISPOT and QFT because of suspected latent or active TB. Of these patients, 318 had results of TST also available. The study observed that the overall agreement with the skin test was similar (ELISPOT κ=0.508, QFT κ=0.460), but fewer BCG-vaccinated individuals were identified as positive by the two blood assays than by the TST (P =0.003 for ELISPOT and P<0.0001 for QFT). Indeterminate results were significantly more frequent with QFT (11%, 43 of 383) than with ELISPOT (3%, 12 of 383; P<0.0001) and were associated with immunosuppressive treatment for both tests. Age younger than five years was significantly associated with indeterminate results with QFT (P= 0.003), but not with ELISPOT. Overall, ELISPOT produced significantly more positive results (38%, n = 144, vs 26%, n = 100, with QFT; P< 0.0001), and close contacts of patients with active TB were more likely to be positive with ELISPOT than with QFT (P= 0.0010). The study concluded that ELISPOT and QFT have higher specificity than the TST. Rates of indeterminate and positive results, however, differ between the blood tests, suggesting that they might provide different results in routine clinical practice. [5] Again, this study was conducted on normal healthy people and not on refugees. The validity of these assays on refugees needs to be tested in future studies.

Effect of Repeated Testing (Reproducibility)

It is a well-reported fact that serial TST can result in a boosting of the skin test result, with the greatest effect seen when the interval between tests is 1-5 weeks; this has been tested in the general and the refugee populations. [19],[20] A recently published study assessed the performance of the QFT in a refugee population relocating to the US. The goals of this study were to evaluate the utility of QFT as a screening tool for LTBI in a refugee population from TB-endemic countries and to assess the reproducibility of the test in the setting of an antecedent TST. In the 198 refugees who were studied, diagnostic agreement between the first QFT and simultaneous TST was 78% (κ= 0.56) and between serial QFT was 89% (κ= 0.76). In serial QFT testing, 70% of subjects had an increased QFT value, perhaps the result of an antecedent TST in the setting of previous TB exposure. This boosting seemed to become less prevalent with time following TST and occurred less frequently in those with negative first QFT readings. Despite small changes in the quantitative results caused by nonspecific variation and boosting, the diagnostic result of the QFT was reliable. The QFT had the potential to replace the TST for LTBI screening in refugees from TB-endemic areas.

Meta Analysis on the Usefulness of IFN- γ Assays in the Screening of Latent TB

This systematic review proposed to estimate sensitivity, specificity and reproducibility of IGRAs (QFT and ELISPOT) for diagnosing latent TB infection in healthy and immune-suppressed persons. The study observed that the pooled specificity was 97.7% (95% CI, 96% to 99%) and 92.5% (CI, 86% to 99%) for QFT and for ELISPOT, respectively. Both assays were more specific than the TST in samples vaccinated with BCG. Both assays had good reproducibility. The review, however, identified the following limitations. Most studies used cross-sectional designs with the inherent limitation of no gold standard for latent TB infection; also, most of them involved small samples with a widely varying likelihood of true-positive and false-positive test results. There is insufficient evidence on IGRA performance in children, immune-compromised persons, and the elderly. The meta-analysis finally concluded that the new IGRAs show considerable promise and have excellent specificity. Additional studies are needed to better define their performance in high-risk populations and in serial testing. Longitudinal studies are needed to define the predictive value of IGRAs. [21]

Latent TB in HIV-Infected Individuals

This study was conducted to evaluate whether interferon-inducible protein (IP)-10, monocyte chemotactic protein (MCP)-2 and inter-leukin (IL)-2 can be useful biomarkers for evaluating a specific response to RD1 antigens associated to latent TB disease in HIV-infected individuals. Sixty-six HIV-infected individuals were prospectively enrolled, 28 with latent TB and 38 without. The results of the study indicate that IP-10 and QFT have a sensitivity of 75% and 85.7%, respectively, in identifying latent TB.

Conclusions

Latent TB is indeed a significant public health problem in refugee camps. When refugees move from a high TB-endemic area to a low endemic area, there is a significant threat for TB spread in this new area. TST, sputum analysis and chest radiography have been routinely used in refugee camps to diagnose TB. Properties like low cost, easy availability, minimal patient discomfort, etc., make TST, sputum analysis and chest radiography the ideal choice in a refugee setting. Also, these techniques have been in use for many years and there are many trained health-care workers who are experts in the interpretation of these results. Elaborate lab facilities are not required for TST, sputum analysis and chest radiography. Studies have clearly shown that chest radiography and sputum analysis have poor validity as screening tools for latent TB. TST is often complicated by false positive results to BCG vaccination, and other mycobacterial infections. Also, all the available data are from the general population and similar data from refugee population is lacking. In spite of these limitations, TST, chest radiography and sputum analysis continue to be used in refugee populations due to financial reasons that are often prevalent in refugee camps.

Two IFN-γ assays namely QFT and ELISPOT show significant promise as effective screening tools for latent TB in refugees. They have good validity and reproducibility. They have been tested to be much better with a higher specificity than TST in refugee populations. They give fewer false positive results than TST. However, they are costly, require lab setting and expertise in interpretation, and their utility in immunosuppressed individuals, children, pregnant women are questionable and are yet to be validated. In HIV-infected individuals, IP-10 antigen assay stands promise for the future. But its effectiveness as a screening tool has not yet been studied.

In conclusion, in a refugee setting where resources are scanty, TST, sputum analysis and chest radiography will continue to be used to test for latent TB not because they are the best but because they are cheap. The poor validity that comes with these tests is inevitable. QFT and ELISPOT are tools that may be used in the future when the cost for these techniques comes down. The other approach that can be adopted is that only patients who test positive with TST may be retested using QFT or ELISPOT. This way, by sequential testing, one can be cost-effective and also the specificity will increase.